如何访问在另一个类中定义的对象？

Question

I have a class called Student, Of course this is not the entire code, I'm just trying to understand the concept. I want to use the another obj in the User class which has college class obj created. The college class is the one that has the other class obj in it. I have only done associations so far not inheritance so solutions related to associations will be helpful.

public class College
{
  private Student[] student;
  private Teacher[] teacher;
  int count;
  public College()
  {
    student = new Student[9];
    teacher = new Teacher[9];
    count = 0;
  }

  public Student[] getStudent()
  {
    return student;
  }

  public void addStudent(Student inStudent)
  {
      student[count] = new Student(inStudent);
      count++;
  }

  public void setStudent(Student[] student)
  {
    this.student = student;
  }

  public Teacher[] getTeacher()
  {
    return teacher;
  }

  public void setTeacher(Teacher[] teacher)
  {
    this.teacher = teacher;
  }

  public boolean equals(Object inObj)
  {
    boolean isEqual = true;
    College inCollege = (College)inObj;

    if(student.length == inCollege.getStudent().length)
    { 
      for(int i = 0; i < student.length; i++)
      {
        if(!student[i].equals(inCollege.student[i]))
        {
            isEqual = false;
        }
      }
    }
    if((teacher.length == inCollege.getTeacher().length) )
    {
      isEqual = false;
      for(int i = 0; i < inCollege.getTeacher().length; i++ )
      {
        if(teacher[i].equals(inCollege.teacher[i]))
        { System.out.println("im in");
          isEqual = true;
        }
      }
    }
    return isEqual;
  }
}

Answer 1

You need to have public accessors (getters) for the private members to be accessed in other classes.

Do it as follows:

import java.util.Scanner;

class Engine {
    String name;
    int year;
    String manufacturer;

    public Engine() {
    }

    public Engine(String name, int year, String manufacturer) {
        this.name = name;
        this.year = year;
        this.manufacturer = manufacturer;
    }

    // getters and setters of instance variables

    @Override
    public String toString() {
        return "Engine [name=" + name + ", year=" + year + ", manufacturer=" + manufacturer + "]";
    }
}

class Submarine {
    private String id;
    private Engine engine;

    public Submarine(String id, Engine engine) {
        this.id = id;
        this.engine = engine;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public void setEngine(Engine engine) {
        this.engine = engine;
    }

    public Engine getEngine() {
        return engine;
    }

    @Override
    public String toString() {
        return "Submarine [id=" + id + ", engine=" + engine + "]";
    }
}

class ShipStorage {
    private Submarine submarine;
    private Submarine[] submarines;

    public void setSubmarine(Submarine submarine) {
        this.submarine = submarine;
    }

    public Submarine getSubmarine() {
        return submarine;
    }

    public void setSubmarines(Submarine[] submarines) {
        this.submarines = submarines;
    }

    public Submarine[] getSubmarines() {
        return submarines;
    }
}

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        ShipStorage store = new ShipStorage();

        // Input submarines
        Submarine[] submarines = new Submarine[3];
        String id, name, manufacturer;
        int year;
        for (int i = 0; i < submarines.length; i++) {
            System.out.print("Enter the ID of the sumarine: ");
            id = in.nextLine();
            System.out.print("Enter the name of its engine: ");
            name = in.nextLine();
            System.out.print("Enter the manufacturing year of its engine: ");
            year = Integer.parseInt(in.nextLine());
            System.out.print("Enter the manufacturer's name of its engine: ");
            manufacturer = in.nextLine();

            submarines[i] = new Submarine(id, new Engine(name, year, manufacturer));
        }

        // Store submarines to ShipStorage
        store.setSubmarines(submarines);

        // Display submarines
        System.out.println("Displaying the data: ");
        for (Submarine submarine : store.getSubmarines()) {
            System.out.println(submarine);
        }
    }
}

A sample run:

Enter the ID of the sumarine: 1
Enter the name of its engine: A
Enter the manufacturing year of its engine: 2010
Enter the manufacturer's name of its engine: X
Enter the ID of the sumarine: 2
Enter the name of its engine: B
Enter the manufacturing year of its engine: 2011
Enter the manufacturer's name of its engine: Y
Enter the ID of the sumarine: 3
Enter the name of its engine: C
Enter the manufacturing year of its engine: 2018
Enter the manufacturer's name of its engine: Z
Displaying the data: 
Submarine [id=1, engine=Engine [name=A, year=2010, manufacturer=X]]
Submarine [id=2, engine=Engine [name=B, year=2011, manufacturer=Y]]
Submarine [id=3, engine=Engine [name=C, year=2018, manufacturer=Z]]

Answer 2

Add a getter to Submarine:

public Engine getEngine() {
     return engine;
}

And same thing to ShipStorage:

public Submarine getSubmarine() {
     return submarine;
}

Then use both of them in User:

Engine = store.getSubmarine().getEngine();

Answer 3

You have a lot of options to do this.

1. Turn private modifier to protected(if classes are in same package) or public of eng object in Submarine class(Do samething in ShipStorage class).However it ignores encapsulation.

  public class Submarine
  {

    public Engine eng;
    public void Submarine ()
    {
      eng = new Engine ();
    }
  }

then use store.submarine[index].engine; statement.

2. Create getter method to get engine instance in Submarine class. Do same thing in ShipStorage class.

  public class Submarine
  {
    private Engine eng;
    public void Submarine ()
    {
      eng = new Engine ();
    }

    public Engine getEngine ()
    {
      return eng;
    }
  }

then use store.getSubmarineArr()[index].getEngine(); statement.

1. You can add public static keyword in front of variable declaration(public static Engine eng;) if you want to access variable without initializing class. However you have a sensible reason to use static keyword. Check use of static method, variable or class.

For instance;

Engine.eng

or

Engine.getEngine();